While it's been installed and used through the wall, the installation
sleeve from outside has vents on the sides. This would seem to
indicate that this air conditioner is a regular window model. I don't
know when wall units were first manufactured (and the date on this
unit is 07/84), so I guess the previous owner was just trying to save
money.


Wall units can have vents on the side. Those side vents just have to be
spaced far enough to allow installation without the vents being blocked by
the the wall. Changing sizes will mean the hole size will have to be
adjusted as well. Were you happy with the 17,500 BTU? Was it too loud or
blow too much cold air? I had one in my home and replaced it with 12,000
BTU. Much quieter and does the job.